1. Field of the Invention
The present invention relates to a light coupling device to couple a light source to an optical fiber, and more specifically to a light coupling device to couple a light source to an optical fiber which is constructed so that the focal length between the light source unit and a GRIN lens, and the focal length between the GRIN lens and the optical fiber end-face, or incident face, can be determined.
2. Prior Art
FIG. 4 shows a schematic diagram of a light coupling device to couple a light source to an optical fiber, which is constructed to feed light power radiated from the light source, i.e., a laser diode, to the optical fiber through a GRIN lens.
In FIG. 4, a can type light source unit 17 is provided, which includes at a central portion thereof a light emitting element 18. A graded index ("GRIN") lens 19 (a lens sold under the trademark "SELFOC" by the Nihon Ita Glass Co., Ltd.) is used to converge the light from the light emitting element 18. In a ferrule 20, there is provided an optical fiber consisting of a core 21 and a clad 22 which is inserted into a central hole 32 of the ferrule and fastened thereto.
A diverging light beam 23 is radiated from the emitting element 18, and includes rays in both the X and Y directions, the different directions corresponding to different magnitudes, respectively. The average radiation angle 2.theta. in each direction is approximately 50 degrees. The light beam is distributed in magnitude in accordance with the Gaussian distribution. The GRIN lens 19 is used to convert the light beam distributed over the wide angle into spot light beam 24 distributed within a small spot, so that the light power output from the GRIN lens 19 is gathered at the end-face of the optical fiber core 21.
A high light coupling efficiency may be attained by aligning the optical axes of the light emitting element 18 in the optical source unit 17, the GRIN lens 19, and the optical fiber core 21, and by optimizing: (1) the focal length f.sub.1 between the end-face of the light emitting element 18 in the light source unit 17 and the GRIN lens 19, and (2) the focal length f.sub.2 between the GRIN lens 19 and the end-face of the optical fiber core 21. Since the core diameter of the single-mode optical fiber measures 9 to 10 .mu.m, the light source unit 17 according to the prior art cannot be precisely fabricated to satisfy the above efficiency requirement.
FIGS. 5A and 5B show the front and side views of light source unit 17 of FIG. 4, respectively. According to the conventional specifications concerning the light source unit 17, as shown in FIGS. 5A and 5B, the diameter of the circular mount of the light source unit 17 has a dimensional allowance of 50 .mu.m. Moreover, the deviation "e" of the light emitting element 18 to the circular mount measures 100 .mu.m between centers. Thus, the light emitting element 18 has a positioning error margin of 150 (=100+50) .mu.m due to the above dimensional deviations. Moreover, positioning error .DELTA. at the emitting end-face of the light emitting element 18 is 100 .mu.m, referring to the center of the circular mount.
The preferable dimensional allowances of the light coupling device are as follows: The alignment error of the optical axes of the light emitting element 18 in the light source unit 17, the GRIN lens 19, and the optical fiber core 21 should be within 2 .mu.m. The error of the focal length f.sub.1 between the end-face of the light emitting element 18 in the light source unit 17 and the GRIN lens 19 should be within .+-.100 .mu.m. The error of focal length f.sub.2 between the GRIN lens 19 and the end-face of optical fiber core 21 should be within .+-.100 .mu.m.
FIG. 6 shows a partial cross-sectional view of a conventional light coupling device. A light source unit 26 of FIG. 6 is not secured to a frame 28 in the upper portion of FIG. 6 (with respect to the center line of the device). However, the light source unit 26 shown in FIG. 6 is secured to the frame 28 in the lower portion of FIG. 6 with respect to the center line of the device. The optical axis of the light source unit 26 is aligned with the optical axis of a GRIN lens 27, and the light source unit 26 is thereafter mounted on the frame 28 by resistive welding. The frame 28 has conical projections 29. The conical projections 29 of the frame 28 are used for resistive welding (see the upper half portion of FIG. 6). When resistive welding is carried out, one of the conical projections 29 is deformed as shown in the lower portion of FIG. 6 so that the conical projection 29 becomes flat. In light coupling devices of the prior art, if the focal length f.sub.1 between the light source unit 26 and the GRIN lens 27 shifts from the calculated value by .DELTA.f as shown in FIG. 6, disadvantageously, no adjustment can be carried out after the welding step.
As further shown in FIG. 6 a sleeve 31 into which a ferrule 30 is inserted is fastened to the frame 28. The light source unit is mounted on the frame after alignment of the optical axes is accomplished.